Kanan was blinded in a lightsaber battle against Maul on Malacore. The cartoon doesn't show you the gruesome details, and Kanan picks up on old mask and wears it after the fateful slash.
Later he is shown with a sort of bandage over his eyes.
Then he is shown with a mask the covers most of his face. It has markings to show where eyes would be.
But in one of the episodes after that, he is shown with the mask removed - and his eyes are just white. He does have a scar across the bridge of his nose, but his eyes look just fine although sightless.
Then he goes back to wearing the facemask again.
Why does he wear the facemask? You would think it is cumbersome and uncomfortable unless it offers some other practical benefit like some sort of magnetic sensor or proximity sensor.